The invention relates to an apparatus for the thickening of brown stock (or bleached stock) pulps from a continuous digester in the lower end of a diffusion washing tower in order to reduce the amount of liquid mixed with the pulp and thereby reduce the amount of liquid extraction necessary in the diffusion washing stages as the pulp moves upwardly in the tower. A conventional diffusion washing tower is shown in U.S. Pat. No. 3,524,551, and in patent application Ser. No. 621,561 filed Oct. 10, 1975 (which is commonly assigned with the present application). Such a structure includes two sets of concentric screens, one set being located above the other, preferably located at the bottom of a diffusion washing tower. The individual screen rings of each set are staggered radially from those of the other set, and the sets are mounted to three or more hollow radial arms which are mounted to an equal number of hydraulic cylinders that reciprocate the screens up and down and provide an exit passageway for removal of the extracted liquid. Conventionally, the liquor extracted through the screen sets is drawn through the common arms and then through the outlets. As the pulp moves upwardly in communication with the upper set of screens, liquor is drawn in the opposite radial direction than it is drawn when passed by the lower set of screens. This opposite liquor drawing performs complete and homogeneous thickening throughout the cross section of the tower before contact of the pulp with the upper diffusion washing stages. The homogeneous cross section thickening is totally dependent upon the rates of liquor extractions at the screen sets, however, and presently there is no control that may be exercise over this homogeneity. The division of extraction flows between the screen sets is quite erratic in nature during conventional operation and the thickening of pulp at the lower screens can reduce the flow through the screen range resulting in an increased flow through the upper screens until thickening on the upper screens causes a reversal of this imbalance, etc.
According to the present invention, the imbalance that results in the prior art may be corrected by separating the flows between the screen sets, and providing two separate suction sources to balance the extraction flow rates of liquid through the first and second screen sets. In this way, a truly homogeneous thickening takes place across the entire cross section of the tower before contact of the pulp with the upper diffusion washing stages.
According to the present invention, means are provided for preventing fluid communication between the interior of screens of the first acreen sets and the interior of screens of the second screen sets, and the withdrawl means comprises first and second withdrawing means, the first withdrawing means for withdrawing screened liquid radially outwardly and only from the first screen set, while the second withdrawing mmeans is for withdrawing screened liquid outwardly (either axially or radially) and only from the second screen set. The second withdrawing means comprises a tubular shaft concentric with the screens of the second screen set and extending axially vertically upwardly from the screens of the second screen set, a stationary withdrawal conduit in fluid communication with the tubular shaft, and bushing means for allowing liquid tight axially slidable engagement between the tubular shaft and the stationary withdrawal conduit. In this way, full advantage may be taken of the entire cross sectional area of conventional withdrawal means while still providing the two separate suction, source withdrawal according to the invention, and it is relatively simple to retrofit existing structures to function to accomplish the results of homogeneous thickening across the entire cross section of a tower. The suction sources associated with the first and second withdrawing means are controlled to balance the extraction flow rates of liquid through the first and second screen sets to provide the desired homogeneity.
It is a primary object of the present invention to provide an improved apparatus for the thickening of pulp in a conventional diffusion washing tower. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.